Broxon and Jesse, in U.S. Pat. No. 2,440,167, discloses a differential ion chamber for separately measuring the flux of slow neutrons and of fast neutron and gamma rays. The apparatus uses three concentric, hollow cylindrical electrodes C.sub.1, C.sub.2 and C.sub.3, defining therebetween an inner subchamber C.sub.1 -C.sub.2 and an outer subchamber C.sub.2 -C.sub.3, each of volume approximately 800 cm.sup.3. C.sub.2 serves as a charged particle collector (first anode), and the inner surface of C.sub.3 and the outer surface of C.sub.2 are each coated with B or Li to emit .alpha. particles when bombarded with slow neutrons. A positive static voltage V(C.sub.1 -C.sub.3)=360-1195 volts is applied across the electrodes C.sub.1 and C.sub.3, and the electrode C.sub.2 is electrically connected to the apparatus casing, with V(C.sub.1 -C.sub.2).apprxeq.360 volts. Each subchamber is filled with an inert gas such as He or Ar at equal pressures p.apprxeq.1 atmosphere. A fast neutron or gamma particle moving in the subchamber gas may produce additional charged particles such as He.sup.+1,2, e.sup.- and negative ions. In subchamber C.sub.1 -C.sub.2 (C.sub.2 -C.sub.3), the electrons and negative ions thus produced move toward C.sub.1 (C.sub.2) due to the imposed potential differences and the positive ions move in the opposite direction. The net electric charge collected at C.sub.2 due to fast neutron or gamma ray reactions in the two subchambers is zero. But the slow neutrons incident on the coated walls of the subchamber C.sub.2 -C.sub.3 will produce excess negative charge at C.sub.2 and allow slow neutron flux to be measured separately from fast radiation particle flux. Net current thus flows from C.sub.2 to C.sub.3. The apparatus uses three electrodes and externally imposed electric potentials between the electrodes to promote charged particle flow. The only nuclear reaction relied upon is apparently n+Li.fwdarw.He.sup.++ (other charged particles), and no excimer reactions are utilized.
A high energy neutron detector is disclosed by Wiegand and Segre in U.S. Pat. No. 2,493,935. A sequence of thin, planar, parallel circular discs of A1, electrically insulated and spaced apart from one another with each disc being coated with a thin Bi layer (surface density.apprxeq.1 mgm/cm.sup.2), is positioned in a closed, hollow chamber containing Ar gas of pressure p.gtorsim.1 atmosphere plus three percent CO.sub.2 gas. Discs number 1, 3, 5, 7, . . . are then connected electrically together, and discs number 2, 4, 6, 8, . . . are connected electrically together; but these two subsets of discs are electrically insulated from one another. An electric voltage V=400-800 volts is externally impressed upon one subset of these discs relative to the other subset. The chamber is apparently provided with a "window" for neutrons to enter in a direction roughly perpendicular to the disc planes. Collisions of fast neutrons of energy E&gt;40 MeV with the Bi atoms in the disc coatings caused Bi to fission, and the high kinetic energy fission fragments cause multiple ionization of the Ar gas particles. The positively charged Ar ions move to the discs of high electric potential, and the negatively charged electrons move to the alternate discs of low electric potential, thus creating an electric current between alternate plates that can be measured and related to the flux of fast neutrons incident upon the apparatus. The Bi coating may be replaced by a coating of Au or Th or other suitable element with different fusion energy thresholds. The wiegand and Segre invention uses coatings of fissionable material, the bulk disc material itself is chosen to possess low capture or reaction cross-section for fast neutrons, and an electric potential is externally imposed on the target discs.
Wiegand discloses a fission indicator in U.S. Pat. No. 2,595,622, using three thin, plane parallel electrodes with the middle electrode being grounded and the two outer electrodes having equal magnitude, opposite sign, externally imposed electrical potential. The three electrodes are positioned in a closed hollow container filled with a noble gas at pressure p.gtorsim.1 atmosphere. The electrode material is a low atomic weight metal, if low energy particles are to be monitored, and is a medium or higher atomic weight metal if high energy particles are to be monitored. The fission fragment particle beam enters the container interior through a thin window in one container wall and causes ionization of the gas between each pair of adjacent electrodes, and the negatively charged particles move toward the higher potential electrode so that a current is established between any two adjacent electrodes. This causes a current to flow in an external circuit connecting the two outer electrodes. The object of the apparatus is to measure only the ionization produced by the fission fragments and to remove or cancel out the effects of ionization from charged particles within the container. The electrode plates are not coated or otherwise treated with a suitable fission material such as Li.sup.6 and no excimer reactions are utilized. The inter-electrode potentials are externally imposed.
U.S. Pat. No. 3,093,567 issued to Jablonski and Laffert, discloses a fission reaction device for generating electric power, by analogy with a thermo-electric cell. A small mass of fissionable material forms the cathode and a metallic or other conducting surface (maintained at a cooler temperature) forms the anode, with the space between cathode and anode being filled with Ar gas at a pressure p.apprxeq.20 Torr. Fission fragments from the cathode material cause ionization of the Ar gas (desired ion density: 10.sup.11 -10.sup.14 ions/cm.sup.3), which neutralizes any space charge adjacent to the (heated) cathode electron-emitting and allows thermionic electron emission thereat according to the Richardson-Dushman equation EQU J(amp/cm.sup.2)=AT.sub.c.sup.2 exp[-e.PHI..sub.c /k.sub.B T.sub.c ]
A=thermionic constant, PA1 .PHI..sub.c =work function of cathode, PA1 T.sub.c =cathode temperature.
With sufficiently high T.sub.c (.gtorsim.2000.degree. K.) and sufficiently low .PHI..sub.c (.ltorsim.1.6 eV), thermionic current density can be of the order of 25 amp/cm.sup.2. The electrical voltage generated between the hot cathode and the cold anode is then V.sub.o =.PHI..sub.c -.PHI..sub.a, where .PHI..sub.a is the electron energy dissipated at the anode as heat; energy loss due to plasma resistance is apparently ignored. The apparatus generates a current with a gap voltage difference of the order of 2 volts. The invention requires use of sufficient fissionable material to produce a self-sustaining fission reaction and to produce a plasma of sufficient charge density to substantially neutralize the space charge that would otherwise develop adjacent to the charge-omitting face of the cathode.
Krieve, in U.S. Pat. No. 3,219,849 discloses a high voltage, low current output electricity generator that uses fissionable material coating for the cathode and that is arranged to minimize axial escape of fission particles and secondary emission electrons. The apparatus includes a coaxial pair of hollow metal cylinders, spaced apart and with either a high vacuum or a low pressure inert gas maintained between the two cylinders. The thin inner cylinder (cathode) is provided with a coating containing fissionable material such as U.sup.235 or P.sup.239. Fission in the cathode coating occurs through interaction with a stream of low energy neutrons incident upon the cathode. The outer cylinder (anode) is sufficiently thick to capture all fission fragments and gamma ray particles incident thereon and is sufficiently thin to be relatively transparent to high energy electrons, created by beta decay or Compton scattering, incident thereon; an anode wall of thickness 0.001 inch of a heavy metal such as Pt or Ni or W is recommended. High energy electrons, emitted within the anode material by beta decay or Compton scattering, are assumed to exit from the anode and to come to rest in the cathode material or other adjacent components. The positively charged fission fragments are assumed to come to rest in the anode material or other adjacent components, thus producing an electrical potential difference between cathode and anode. A coil surrounding the cathode and anode cylinders produces an axial magnetic field that tends to deflect and return charged particles emitted from the cathode to that cylinder. The inert gas particles, if any, contained between cathode and anode are ionized by collisions with the energetic fission fragments that move from the cathode toward the anode; the positively charged inert gas ions also move toward the anode, thus increasing the current in that direction. The average fission fragment is assumed to lose most or all of its kinetic energy to ionizing collisions with the inert gas particles, thus increasing the number of positively charged particles available to move to the anode. The Krieve apparatus uses an external magnetic field for charged particle control and may not be suitable for generation of high cathode-anode current, which could be limited by space charge effects.